They’re questioning & answering themselves that they are the greatest.

Coming from Nas, “we major” sounds like “we made you.” This implies that the greats in the game end up shaping future generations of rappers. Given the stage of Kanye’s career at the time this song was recorded — it could be taken that Nas is inferring that his influence is notable in Kanye’s music — why else would Nas be on the song if ‘Ye wasn’t a fan? (considering the Jay-Z/Nas feud at the time, and the fact that Kanye produced Takeover).

Also, by saying “we major” Kanye and Nas are saying that rap is their speciality; in other words, Kanye and Nas are saying that they “majored” in rap.